Push-pull amplifiers have been used in the art for a number of years Generally, a first "push" transistor provides positive output current in response to a positive input signal and a second "pull" transistor provides negative output current in response to a negative input signal. Due to switching characteristics of the output transistors, it is commonly desirable to bias the output transistors such that the effect of any threshold voltages which are necessary before conduction can be minimized or, preferably eliminated.
Prior art biasing techniques fall generally in one of two categories. The first category includes those circuits which provide a constant bias current regardless of operational conditions. The second category includes those circuits whose bias current is dependent upon a voltage source which may or may not be thermally linked to the output transistors. Both categories of biasing circuits suffer from the disadvantage that their performance is degraded by operational and physical characteristics of the circuit components Particularly, the bias current which is established by these circuits is not stable with varying temperature. The temperature characteristics of these circuits, as well as other undesired operational characteristics, result in increased power dissipation and/or increased distortion in the output signal.
It is desirable, therefore, to have a biasing circuit for use with a push-pull amplifier which establishes the bias current substantially independent of variations in temperature and other operating conditions. It is further desirable to provide a biasing circuit which minimizes power dissipation while minimizing any penalties in distortion of the output signal.